# Homework Help: 3 Forces on a Ring

1. Jan 28, 2012

### zaper

Three forces are applied to a ring (as shown in the photo) that lies on a frictionless surface in the xy plane. The ring has a mass of 100 kg. Fa=200N, Fc=240N and the angle between Fa and Fb is 135°.

What is Fb if:

The system is stationary?

The system accelerates at .5 m/s2?

For some reason I just can't get a grasp on this problem. I understand that to be stationary all forces must cancel out, but I can't figure out how with Fc having a stronger pull in the x direction than Fa how Fb (which appears to go straight down) can stop Fc.

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• ###### phys ring prob.jpg
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Last edited: Jan 28, 2012
2. Jan 28, 2012

### tiny-tim

hi zaper!
you aren't told the angle between Fc and the other two …

so you can make it anything you like!

3. Jan 28, 2012

### zaper

So Basically to not move I should make the angle between Fb and Fc 135 as well which means that Fa and Fc will cancel x-wise and Fb will have to be 2*Fa*cos(45)?

4. Jan 28, 2012

### tiny-tim

no, because Fa is 200 N and Fc is 240 N

5. Jan 28, 2012

### zaper

Oh yeah. So then 200cos(45)=240cos(x) so Fc is at 53.9. This means Fb=200sin(45)+240sin(53.9)which is 335.3

Last edited: Jan 28, 2012
6. Jan 28, 2012

### tiny-tim

no, not 45°, you need to use an unknown θ

7. Jan 28, 2012

### zaper

Yeah I edited my previous post so hopefully it's correct. I'm sorry. My brain is not working this morning

8. Jan 28, 2012

### tiny-tim

ahh!
yes! (i haven't checked the figures, but …) that method looks fine

9. Jan 28, 2012

### zaper

Ok so that solves the first part. Now for the second part since it goes .5m/s2 in the x direction and the ring is 100 kg then Fc is 50 N greater than Fa in the x direction so 200cos(45)=240cos(x)-50?

10. Jan 28, 2012

### zaper

If this method is correct then I get that Fc is at 37.1. This means that Fb=200sin(45)+240sin(37.1) which is 286.2

11. Jan 28, 2012

### tiny-tim

yes, but you'll need a y equation also

12. Jan 28, 2012

### zaper

I have that in my last post I believe

13. Jan 28, 2012

### tiny-tim

ahh!
yes, that looks fine too